DE 10 2008 033 048 A1 discloses a field device for a process automation system which is equipped with a device for locally acquiring electrical power. In addition to or as an alternative to supplying electrical power in a two conductor bus, electrical energy is generated locally in the field device by converting an operating pressure applied there by means of a piezo-element arrangement. The piezo-element arrangement is designed as a spring mounted piezo-electric patch transducer that is capable of vibrating. This solution for locally generating electrical energy directly in the field device assumes, however, that the operating pressure is constantly available as an energy source. This also restricts the area of application of this technical solution to pressure-operated field devices.
DE 200 238 65 U1 discloses an additional technical solution to the local supply of field devices with electrical power, in which an additional power supply unit is provided which can be electrically connected to the field device via a service socket. The power supply unit includes at least one battery. As an alternative to or in conjunction with the at least one batter, it is further proposed to use, for example, solar cells, a Peltier element, a vibrational energy transducer or a rotational energy transducer.
However, using a conventional battery requires replacement of the battery once it become exhausted. If the battery is in the form of an accumulator, the battery needs to be recharged. A battery status indication or the like is then also necessary. By contrast, while solar cells and the other means for supplying power use locally renewable energy using photovoltaic or electrothermal or electromechanical conversion, the utilization of these forms of energy is dependent on the presence of light, heat or vibrations. Since field devices often need to be operated in explosion-hazardous environments, care must also be taken that these field devices themselves, as well as their power supply units, are designed to be intrinsically safe according to the relevant safety regulations.